Field of the Invention
This invention relates to a surface acoustic wave device, and more particularly to such a device in which influences by mechanical reflections of surface acoustic waves are reduced.
A surface acoustic wave device mainly consists of a piezoelectric substrate made of a piezoelectric single crystal material such as lithium niobate (LiNbO.sub.3), a piezoelectric ceramic material or alternatively a combination of a non-piezoelectric plate and a piezoelectric film deposited thereon, for example, so as to convert an electric signal to a surface acoustic wave by means of a transducer provided on the piezoelectric substrate and to propagate the surface acoustic wave along the surface of the substrate. It is now employed as filters and other various electronic parts.
FIG. 1 shows a filter as an example of such electronic parts in which reference numeral 1 designates a piezoelectric substrate. Reference numeral 2 designates an input transducer consisting of a pair of interdigitating comb-shaped electrodes 2A and 2B. Reference numeral 3 denotes an input transducer consisting of a pair of interdigitating comb-shaped electrodes 3A and 3B. When an electric signal is applied to an input terminal IN, it is converted to a surface acoustic wave by means of the input transducer 2 and travels along the surface of the piezoelectric substrate 1 as shown by the arrow in the Figure. When the surface acoustic wave reaches the output transducer 3, it is reconverted to an electric signal and is taken out from an output terminal OUT. The comb-shaped electrodes 2A, 2B and 3A, 3B are so-called normalized electrodes wherein each electrode finger width W and each space L between respective adjacent interdigitating electrode fingers are .lambda..sub.0 /4, respectively, when .lambda..sub.0 is the wavelength of the center frequency f.sub.0 of a surface acoustic wave to be employed.
The transducers consisting of the comb-shaped electrodes with said finger width and space, however, cause mechanical reflections such as multiple reflections i.e. so-called T.T.E.(triple transit echoes) of a surface acoustic wave, thereby worsening the wave passing-through effect of the filter, for example.
To prevent such a mechanical reflection, it is known to, as shown in FIG. 2, divide each finger of the comb-shaped electrodes 2A, 2B, 3A and 3B into two parts so that each divided electrode finger width W and each space L between the respective adjacent interdigitating electrode fingers become .lambda..sub.0 /8, respectively. It is called a double electrode transducer. In the double electrode transducer, since phases of reflected waves at the tips of the respective electrode fingers are different by 180.degree. so as to be opposite phases, the reflected waves counteract each other.
However, the double electrode tranducer requires a high working accuracy because higher the frequency, smaller the wavelength .lambda..sub.0 becomes. Further, the double electrode transducer is liable to cause short circuit between opposed electrodes cr breakdown of electrodes, thereby leading to a lower productivity of the device.
To overcome the drawbacks, single-phase transducers as shown in FIGS. 3(a) and 3(b) have been proposed. This structure employs the piezoelectric substrate 1 comprising an elastic plate 4 and a piezoelectric film 5 deposited thereon. A lower electrode 6 to serve as one counterpart of the comb-shaped electrodes is made on the elastic plate 4 and under the piezoelectric film 5 while upper electrodes 7A and 7B each to serve as the other counterpart of the comb-shaped electrodes are made on the piezoelectric film 5 so as to be opposed to the lower electrode 6. Each electrode finger width W and each space L between the electrode fingers are .lambda./2, respectively.
With this arrangement, by connecting a signal source 8 to the upper electrode 7A and the lower electrode 6, generating and propagating a surface acoustic wave and taking out the resulting electric signal from an end of the load 9 connected between the upper electrode 7B and the lower electrode 6, it is possible to prevent short circuit between the opposed counterparts of the electrodes and to relax their working accuracy, thereby leading to a better productivity of the device.
However, this construction does not allow a driving of the transducers employing balance power supply from the power source 8. Therefore, influences by electric signals which are not converted to surface acoustic waves and travel as they are direct waves (so-called feedthrough) cannot be cancelled, thereby worsening the wave passing-through effect of the device.
To overcome the drawback, further, it has been proposed to oppose two pieces of single phase transducers in parallel with each other. More specifically, further upper comb-shaped electrodes 8A and 8B are added and are opposed to the aforementioned upper electrodes 7A and 7B at .lambda..sub.0 /2 phase difference and at a space a. This construction permits a driving of the transducers by balance power supply by applying an electric signal at 180.degree. phase difference to the upper electrodes 7A and 7B from the power source 8 via a balance power supply transformer T, thereby reducing influences by the feedthrough.
The construction, however, cannot give a complete solution because the space a to prevent short circuit between the opposed electrodes causes turbulence of the waveform of the composite wave of surface acoustic waves S.sub.1 and S.sub.2 excited by the electrodes 7A and 8A, respectively.